Many sensitive systems such as measurement systems require controlled environments in order to ensure stability and repeatability, as well as system to system matching. An example thereof is an interferometric measurement system such as NanoMapper or WaferSight (both measurement systems made by KLA-Tencor Corporation.
Environmental control and stability for sensitive systems is best provided by creating a stable mini-environment which encloses the system. Examples of controlled mini-enclosures for use as discussed above are described in:                1. U.S. Pat. No. 5,195,922, issued March 1993 discloses a modular chamber which encloses the work pieces and processing instruments to isolate them from the others in the room. The chamber creates a mini environment that permits control over particulate contamination and individualized regulation from others within a single room.        2. U.S. Pat. No. 5,960,638, issued October 1999, discloses a mall environmental control unit (ECU) for mini-enclosures. Each ECU has the capability of controlling temperature, humidity, and airflow rate delivered into the enclosure. It is mounted on the top of individual mini-enclosure to make the enclosure have its own environmental control.        3. U.S. Pat. No. 6,632,260, issued October 2003, discloses a clean gas flow enclosure that provides for the isolation of material from airborne micro-particulate contamination. The enclosure is able to adjust the clean gas flow in different directions by the pressure differential distribution technology.        
Though each of the above cited systems provides some aspects of environmental control, none of the systems provides all aspects of the control required for sensitive systems such as interferometric measurement systems.